The present invention relates generally to a resistor for a thermal flowmeter, and more specifically to a resistor for a thermal flowmeter adapted to measure an air intake in internal combustion engines.
So far, a thermal air flowmeter has been known as a device for measuring the flow rate of air in an automotive internal combustion engine. According to this flowmeter, a heat generator is located in an air intake passage and maintained at a certain temperature, e.g., 100.degree. C., by current control to measure a change in the quantity of heat needed to cool the heat generator in the form of a resistance value change.
Such a thermal flowmeter includes an electric circuit such as one shown in FIG. 1. As illustrated, a heat-generation resistor RH and a temperature-compensation resistor R.sub.C are located in an air pipe 1 for contact with an air flow through it, and resistors R.sub.1 and R.sub.2, located on the outside of an air flow passage 2, are connected thereto in the form of a bridge circuit. Here, the heat-generation resistor R.sub.H is a resistor for flow rate measurement while the temperature-compensation resistor R.sub.C is a resistor that is maintained at the same temperature as that of air in the air flow passage 2. For R.sub.H and R.sub.C, resistors having resistance values of about 10-30.OMEGA. and about 150-1,000.OMEGA. are used, respectively. In FIG. 1, reference numeral 3 stands for a transistor, 4 a comparator and 5 a terminal to which the voltage for driving the electric circuit is applied. The bridge circuit is balanced so as to make the temperature of the resistor R.sub.H higher than that of air (i.e., the temperature of the resistor R.sub.C), whereby a heating current is supplied to the bridge to keep the temperature of the resistor R.sub.H constant. Then, the voltages at both ends of R.sub.1 can produce an output 6 that corresponds to the flow rate of air.
A typical example of the resistor for a thermal flowmeter, mentioned above as is referred to in JP-A-61-40513 which discloses a cylindrical thin film form of platinum resistor having a temperature coefficient of resistance of 3,500 ppm/.degree.C. with a deviation of .+-.3% (=.+-.105 ppm/.degree.C.).
However, this thin film form of platinum resistor has (1) a temperature coefficient of resistance of 3,500 ppm/.degree.C. and (2) a temperature-coefficinet-of-resistance deviation as large as .+-.105 ppm/.degree.C.
If there is a large deviation in the temperature coefficient of resistance of such a thin film form of platinum resistor, there is then a flow rate measurement error depending on the temperature of a fluid. In order to make temperature compensation depending on this deviation in the temperature coefficient of resistance, therefore, it is required to regulate an electric circuit. Explaining this with reference to the above-mentioned conventional thin film form of platinum resistor having a deviation of .+-.100 ppm/.degree.C. in the temperature coefficient of resistance, there is a fluctuation of at most .+-.8% in the output of each flowmeter depending on the temperature of fluid is at (e.g., 20.degree. C. and 80.degree. C.), even when the electric circuit is pre-adjusted so as to permit the flowmeter to produce a constant output at normal temperatures (e.g., 20.degree. C.). The compensation of this requires troublesome circuit regulation, because the electric circuit must be regulated for temperature compensations at the two temperatures, i.e., normal and high temperatures (e.g., 20.degree. C. and 80.degree. C.).
The large deviation in the temperature coefficient of resistance of the above-mentioned conventional resistor is caused by the fact that the platinum thin film is not uniformly formed on a cylindrical ceramic member, resulting in a local drop of its temperature coefficient of resistance.
An object of the invention is therefore to provide a resistor for a thermal flowmeter, which enables the above-mentioned deviation in the temperature coefficient of resistance to be effectively reduced or eliminated, thus making circuit regulation operation easier.